Read threshold setting based on temperature integral

ABSTRACT

A method for data storage in a memory that includes multiple analog memory cells fabricated using respective physical media, includes identifying a group of the memory cells whose physical media have deteriorated over time below a given storage quality level. A rejuvenation process, which causes the physical media of the memory cells in the group to meet the given storage quality level, is applied to the identified group. Data is stored in the rejuvenated group of the memory cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/649,382, filed Dec. 30, 2009, which claims the benefit ofU.S. Provisional Patent Application 61/141,842, filed Dec. 31, 2008,U.S. Provisional Patent Application 61/234,688, filed Aug. 18, 2009,U.S. Provisional Patent Application 61/243,726, filed Sep. 18, 2009,U.S. Provisional Patent Application 61/244,500, filed Sep. 22, 2009, andU.S. Provisional Patent Application 61/251,787, filed Oct. 15, 2009,whose disclosures are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to memory devices, andparticularly to methods and systems for rejuvenating analog memorycells.

BACKGROUND OF THE INVENTION

Several types of memory devices, such as Flash memories, use arrays ofanalog memory cells for storing data. Each analog memory cell stores aquantity of an analog value, also referred to as a storage value, suchas an electrical charge or voltage. This analog value represents theinformation stored in the cell. In Flash memories, for example, eachanalog memory cell holds a certain amount of electrical charge. Therange of possible analog values is typically divided into intervals,each interval corresponding to one or more data bit values. Data iswritten to an analog memory cell by writing a nominal analog value thatcorresponds to the desired bit or bits.

Some memory devices, commonly referred to as Single-Level Cell (SLC)devices, store a single bit of information in each memory cell, i.e.,each memory cell can be programmed to assume either of two possibleprogramming levels. Higher-density devices, often referred to asMulti-Level Cell (MLC) devices, store two or more bits per memory cell,i.e., can be programmed to assume more than two possible programminglevels.

SUMMARY OF THE INVENTION

An embodiment of the present invention provides a method for datastorage, including:

in a memory that includes multiple analog memory cells fabricated usingrespective physical media, identifying a group of the memory cells whosephysical media have deteriorated over time below a given storage qualitylevel;

applying to the identified group a rejuvenation process, which causesthe physical media of the memory cells in the group to meet the givenstorage quality level; and

storing data in the rejuvenated group of the memory cells.

In an embodiment, applying the rejuvenation process includes causingremoval of electrical charge that is trapped in the physical media ofthe memory cells in the group. In a disclosed embodiment, applying therejuvenation process includes inhibiting programming of the memory cellsin the group for a given time period.

In some embodiments, applying the rejuvenation process includes heatingthe memory cells in the group. Heating the memory cells may includeactivating a heating device that is thermally coupled to the group ofthe memory cells. Alternatively, heating the memory cells may includeapplying memory access operations to some of the memory cells in thememory. Further alternatively, heating the memory cells may includepreventing heat removal from the group. In an alternative embodiment,heating the memory cells includes directing to the memory cells in thegroup heat generated by another component that is not part of the group.

In another embodiment, applying the rejuvenation process includesattempting to erase or program the memory cells in the group. In yetanother embodiment, applying the rejuvenation process includes copyingthe data from the group to an alternative location in the memory beforerejuvenating the group. In a disclosed embodiment, applying therejuvenation process includes heating the memory cells in the groupwhile periodically refreshing the data stored in the memory cells in thegroup. Periodically refreshing the data may include refreshing the dataduring the rejuvenation process at a rate that is higher than a nominalrefresh rate used for refreshing the data in the memory.

In some embodiments, identifying the group includes programmingrespective storage values into the memory cells in the group, measuringa shift that develops in the programmed storage values over a predefinedtime period, and determining, based on the measured shift, that thephysical media have deteriorated below the given storage quality level.In an embodiment, the method includes copying the data from the group toanother group of memory cells before programming the respective storagevalues into the memory cells in the group.

In some embodiments, identifying the group includes making a predictionthat the physical media of the group of the memory cells is expected todeteriorate below the given storage quality level, so as to apply therejuvenation process responsively to the prediction. In an embodiment,making the prediction includes estimating an integral of a temperatureof the memory cells in the group over time, and initiating therejuvenation process responsively to the integral. In anotherembodiment, making the prediction includes estimating one or morestatistical properties of analog values stored in the memory cells inthe group, and initiating the rejuvenation process responsively to theestimated statistical properties. Estimating the statistical propertiesmay include predicting a number of errors in the group of the memorycells based on the statistical properties.

There is additionally provided, in accordance with an embodiment of thepresent invention, apparatus for data storage, including:

an interface for communicating with a memory that includes multipleanalog memory cells fabricated using respective physical media; and

circuitry, which is configured to identify a group of the memory cellswhose physical media have deteriorated over time below a given storagequality level, to apply to the identified group a rejuvenation process,which causes the physical media of the memory cells in the group to meetthe given storage quality level, and to store data in the rejuvenatedgroup of the memory cells.

There is also provided, in accordance with an embodiment of the presentinvention, a method for data storage, including:

in a memory that includes multiple analog memory cells, storing data ina group of the analog memory cells by writing respective storage valuesinto the memory cells in the group;

estimating an integral of a temperature of the memory cells in the groupover time;

setting at least one read threshold for reading the memory cells in thegroup responsively to the estimated integral; and

reading the analog storage values from the memory cells in the groupusing the at least one read threshold, so as to reconstruct the storeddata.

There is further provided, in accordance with an embodiment of thepresent invention, apparatus for data storage, including:

an interface for communicating with a memory that includes multipleanalog memory cells; and

circuitry, which is configured to store data in a group of the analogmemory cells by writing respective storage values into the memory cellsin the group, to estimate an integral of a temperature of the memorycells in the group over time, to set at least one read threshold forreading the memory cells in the group responsively to the estimatedintegral, and to read the analog storage values from the memory cells inthe group using the at least one read threshold, so as to reconstructthe stored data.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method for data storage, including:

operating a memory, which includes a plurality of analog memory cellsand generates internal heat;

heating at least some of the memory cells by applying to the memoryexternal heat, in addition to the internal heat generated by the memory;and

storing data in the heated memory cells.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that schematically illustrates a memorysystem, in accordance with an embodiment of the present invention;

FIG. 2 is a flow chart that schematically illustrates a method forrejuvenating analog memory cells, in accordance with an embodiment ofthe present invention;

FIGS. 3 and 4 are schematic, pictorial illustrations of memory systemsthat employ memory device heating, in accordance with embodiments of thepresent invention;

FIG. 5 is a flow chart that schematically illustrates a method forrejuvenation and in-place data refreshing of analog memory cells, inaccordance with an embodiment of the present invention; and

FIG. 6 is a flow chart that schematically illustrates a method forrejuvenating analog memory cells, in accordance with an alternativeembodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Analog memory cells of a certain type are fabricated using certainphysical media. For example, NAND Flash memory cells are typicallyimplemented using floating-gate transistors that are disposed on asemiconductor substrate. Other types of non-volatile memory cells areimplemented using other semiconductor configurations. Extensive use ofanalog memory cells sometimes causes aging effects in the cells'physical memory media, which causes the storage quality of the memorycells to deteriorate. In NAND Flash memory cells, for example, chargetrapping in the boundary regions between memory cells decreases floatinggate isolation, and may cause read errors and increase the memory cells'sensitivity to interference and charge loss. In known memory managementschemes, when the deterioration of the physical memory media in a givengroup of memory cells becomes intolerable, the group is marked as faultyand taken out of service. Thus, the available storage capacity of amemory device decreases over time, and at some point drops below itsspecified capacity.

Embodiments of the present invention that are described hereinbelowprovide improved methods and systems for managing memory devicescomprising analog memory cells. The disclosed methods and systems applyvarious rejuvenation processes to memory cells whose physical memorymedia have deteriorated below a tolerable level. Rejuvenation mayinvolve, for example, heating the memory cells or inhibiting programmingof the memory cells for certain time periods. The techniques describedherein return the storage quality of the rejuvenated memory cells to atolerable level, so that the memory cells can be put back in service.Rejuvenation can be applied at different granularities, such as toindividual groups of memory cells in a semiconductor die, individualdies within a multi-die memory device, individual memory devices withina multi-device assembly, or even individual memory assemblies (e.g.,solid state disks) in a storage system. Several examples of memorymanagement schemes that use rejuvenation processes are also describedbelow.

In order to understand the disclosed techniques, it is important todifferentiate between (1) deterioration of the physical memory media ofthe memory cells and (2) distortion or impairments in the analog valuesstored in the memory cells. It is similarly important to distinguishbetween (1) rejuvenation of the physical memory media of the memorycells and (2) refreshing of the analog values stored in the memorycells. The methods and systems described herein are concerned withrepairing the deterioration of the physical memory media (e.g.,eliminate charge traps in the oxide between and/or below the memorycells), irrespective of analog values or data that may be stored in thememory cells. Impairments in the analog values that represent the datastored in the memory cells, e.g., cell charge levels or thresholdvoltages, can be corrected by other means, which are beyond the scope ofthe present disclosure.

When using the disclosed rejuvenation techniques, memory cell groupsthat would otherwise be declared irreversibly faulty can continue tostore data reliably. As a result, the overall storage capacity of amemory system or device decreases over time at a slower rate, and itseffective lifetime is thus extended. Moreover, the disclosed techniquesenable the design of memory systems and devices with less sparecapacity, and therefore the size and cost of memory systems and devicescan be reduced.

System Description

FIG. 1 is a block diagram that schematically illustrates a multi-devicememory system 20, in accordance with an embodiment of the presentinvention. System 20 accepts data for storage from a host 24 and storesit in memory, and retrieves data from memory and provides it to thehost. In the present example, system 20 comprises a Solid-State Disk(SSD) that stores data for a host computer. In alternative embodiments,however, system 20 may be used in any other suitable application andwith any other suitable host, such as in computing devices, cellularphones or other communication terminals, removable memory modules suchas Disk-On-Key (DOK) devices, Secure Digital (SD) cards, Multi-MediaCards (MMC) and embedded MMC (eMMC), digital cameras, music and othermedia players and/or any other system or device in which data is storedand retrieved.

System 20 comprises multiple memory devices 28, each comprising multipleanalog memory cells. In the present example, devices 28 comprisenon-volatile NAND Flash devices, although any other suitable memorytype, such as NOR and Charge Trap Flash (CTF) Flash cells, phase changeRAM (PRAM, also referred to as Phase Change Memory—PCM), Nitride ReadOnly Memory (NROM), Ferroelectric RAM (FRAM), magnetic RAM (MRAM) and/orDynamic RAM (DRAM) cells, can also be used.

In the context of the present patent application and in the claims, theterm “analog memory cell” is used to describe any memory cell that holdsa continuous, analog value of a physical parameter, such as anelectrical voltage or charge. Any suitable type of analog memory cells,such as the types listed above, can be used. In the present example,each memory device 28 comprises a non-volatile memory of NAND Flashcells. The charge levels stored in the cells and/or the analog voltagesor currents written into and read out of the cells are referred toherein collectively as analog values or storage values. Although theembodiments described herein mainly address threshold voltages, themethods and systems described herein may be used with any other suitablekind of storage values.

System 20 stores data in the analog memory cells by programming thecells to assume respective memory states, which are also referred to asprogramming levels. The programming levels are selected from a finiteset of possible levels, and each level corresponds to a certain nominalstorage value. For example, a 2 bit/cell MLC can be programmed to assumeone of four possible programming levels by writing one of four possiblenominal storage values into the cell. The memory cells are typicallyarranged in rows and columns. Typically, a given memory device comprisesmultiple erasure blocks (also referred to as memory blocks), i.e.,groups of memory cells that are erased together.

Each memory device 28 may comprise a packaged device or an unpackagedsemiconductor chip or die. A typical SSD may comprise a number of 4 GBdevices. Generally, however, system 20 may comprise any suitable numberof memory devices of any desired type and size. Although the systemconfiguration of FIG. 1 comprises multiple memory devices, some of themethods and systems described herein can also be used in systems havingonly a single memory device.

System 20 comprises a memory controller 32, which accepts data from host24 and stores it in memory devices 28, and retrieves data from thememory devices and provides it to the host. Memory controller 32comprises a host interface 36 for communicating with host 24, a memoryinterface 40 for communicating with memory devices 28, and a processor44 that processes the stored and retrieved data. In some embodiments,controller 32 encodes the stored data with an Error Correction Code(ECC). In these embodiments, controller 32 comprises an ECC unit 48,which encodes the data before stored in devices 28 and decodes the ECCof data retrieved from devices 28. The functions of processor 44 can beimplemented, for example, using software running on a suitable CentralProcessing Unit (CPU), using hardware (e.g., state machine or otherlogic), or using a combination of software and hardware elements.

Memory controller 32, and in particular processor 44, may be implementedin hardware. Alternatively, the memory controller may comprise amicroprocessor that runs suitable software, or a combination of hardwareand software elements. In some embodiments, processor 44 comprises ageneral-purpose processor, which is programmed in software to carry outthe functions described herein. The software may be downloaded to theprocessor in electronic form, over a network, for example, or it may,alternatively or additionally, be provided and/or stored on tangiblemedia, such as magnetic, optical, or electronic memory.

The system configuration of FIG. 1 is an example configuration, which isshown purely for the sake of conceptual clarity. Any other suitablememory system configuration can also be used. Elements that are notnecessary for understanding the principles of the present invention,such as various interfaces, addressing circuits, timing and sequencingcircuits and debugging circuits, have been omitted from the figure forclarity.

In the exemplary system configuration shown in FIG. 1, memory devices 28and memory controller 32 are implemented as separate Integrated Circuits(ICs). In alternative embodiments, however, the memory devices and thememory controller may be integrated on separate semiconductor dies in asingle Multi-Chip Package (MCP) or System on Chip (SoC), and may beinterconnected by an internal bus. Further alternatively, some or all ofthe memory controller circuitry may reside on the same die on which oneor more of the memory devices are disposed. Further alternatively, someor all of the functionality of memory controller 32 can be implementedin software and carried out by a processor or other element of the hostsystem, or by any other type of memory controller. In some embodiments,host 24 and Memory controller 32 may be fabricated on the same die, oron separate dies in the same device package.

Automatic Rejuvenation of Analog Memory Cells

The analog memory cells in memory devices 28 are fabricated usingcertain physical media. In the present example, devices 28 comprise NANDFlash devices, and the analog memory cells comprise floating-gatetransistors that are disposed on a semiconductor substrate. Inalternative embodiments, memory devices 28 may comprise any other typeof analog memory cells, which are implemented using any other suitablephysical media.

The physical memory media of the analog memory cells is typicallyspecified and designed to provide a certain level of storage quality.The storage quality can be defined in any suitable way, such as thenumber of errors found per page during read operation, the maximalretention time during which that the data hold, or using any othersuitable quality metric. In practice, however, the storage quality leveloften deteriorates with continued use of the memory cells. Thisdeterioration is sometimes referred to as cell wearing or aging. In NANDFlash memory cells, for example, charge traps are gradually formed inthe boundary regions between the memory cells (sometimes referred to asinter-cell oxide) and/or in the oxide that isolates the floating gatefrom the device channel (sometimes referred to as bottom oxide or tunneloxide). The charge traps decrease the isolation of the cells' floatinggates, and as a result the memory cells become more sensitive tointerference (e.g., disturb noise) and charge loss. Other physical mediatypes used in other memory cell types suffer from other kinds of storagequality deterioration.

At some point in time, the storage reliability of a certain group ofmemory cells may drop below a certain tolerable level, and the cellgroup may be regarded as unsuitable for storing data. In known memorymanagement schemes, when the deterioration of the physical memory mediain a given group of memory cells becomes intolerable, the group istypically marked as faulty and taken out of service. Thus, the availablestorage capacity of a memory device decreases over time, and at somepoint drops below its specified capacity.

Embodiments of the present invention provide methods and systems thatautomatically rejuvenate the physical memory media of analog memorycells in device 28. The rejuvenation methods described herein improvethe storage reliability of the physical media, bringing it back to atolerable level. Once rejuvenated, a group of memory cells can be putback in service and used for subsequent data storage. As a result, theoverall storage capacity of system 20 decreases at slower rate, and itseffective lifetime is extended.

FIG. 2 is a flow chart that schematically illustrates a method forrejuvenating analog memory cells, in accordance with an embodiment ofthe present invention. The method begins by defining a certain storagequality level as tolerable, at a quality definition step 50. As notedabove, the storage quality characterizes the physical memory media ofthe memory cells, and not the analog values or data that may or may notbe stored in the memory cells.

Various criteria and conditions can be defined for deciding whether agroup of memory cells has an intolerable storage quality, and shouldtherefore be rejuvenated. For example, a memory block may be regarded acandidate for rejuvenation if the shift in the distribution of thecells' analog storage values (e.g., threshold voltages) over apredefined time period exceeds a certain tolerable level. As anotherexample, a memory block may be regarded a candidate for rejuvenation ifit fails to erase properly. A memory block may be regarded as faulty,for example, following a single erase failure, following a given numberof consecutive erase failures, or following M erase failure out of Nattempts. As another example, a group of memory cells (e.g., a memoryblock or page) may be regarded as having insufficient storage quality ifit fails to program correctly, e.g., following a single programmingattempt, several programming attempts, or M programming failures out ofN attempts.

As yet another example, a group of memory cells can be regarded as acandidate for rejuvenation if a high number of read errors areencountered when reading data from this group. In an example embodiment,the memory controller may run a background task that reads pages fromvarious memory blocks according to a certain pattern (e.g., reads theNth page from each block, and later the N+1th page from each block,etc., or reads randomly-selected pages from the blocks). The memorycontroller may alternatively read parts of pages (e.g., sectors), readfrom only a subset of the blocks, or read different pages from differentblocks. For each read operation, the memory controller applies ECCdecoding, and counts the number of errors corrected in each page. Basedon the numbers of corrected errors, the memory controller assesses thehealth status of each memory block.

Alternatively, any other suitable quality level or criterion can beused. The storage quality is also sometimes referred to as the healthlevel of the memory cell group.

Memory controller 32 selects a certain group of memory cells, such as amemory block, a memory die or a packaged multi-die memory device, at agroup selection step 54. The memory controller assesses the storagequality of the selected cell group, at a quality assessment step 58. Forexample, the memory controller may attempt to erase or program some orall of the memory cells in the group in order to assess whether theselected cell group is functional or faulty.

As another example, the memory controller may test a given group ofmemory cells by programming the memory cells, reading the memory cellsafter a predefined time period, and measuring the shift that developedin the distribution of the cells' analog storage values (e.g., thresholdvoltages) during this time period. If the shift is larger than a certaintolerable value, the controller may select this memory cell group as acandidate for rejuvenation. When using this technique, the memorycontroller may program the memory cells in the group with dummy data, orwith user data as part of the normal storage process.

As can be appreciated, when the memory controller applies theabove-described technique, it should refrain from programming or erasingthe cell group in question during the predefined time period. In someapplications, however, the data in this cell group needs to be updatedor erased frequently. Thus, in some embodiments the memory controllerfirst copies the data from the group to another memory location beforetesting it. When the group comprises a memory block, for example, thememory controller may copy the block content to another block, removethe block temporarily from the pool of available blocks, and then applythe above-described quality testing process. Based on the testingresult, the memory controller can decide whether to return the block tothe pool of available blocks or rejuvenate it.

Memory controller 32 checks whether the assessed storage quality of theselected cell group meets the tolerable quality level defined at step 50above, at a checking step 62. If the storage quality is sufficient, themethod loops back to step 54, in which the memory controller selectsanother group of memory cells for evaluation.

If, on the other hand, the assessed storage quality is insufficient, thememory controller initiates a rejuvenation process, which improves thestorage quality of the physical media of the memory cells in the group.Several examples of rejuvenation processes are described further below.In some embodiments, the memory controller moves data that is stored inthe cell group to an alternative location before rejuvenation, at acopying step 66. The memory controller may move the data, for example,to another block, die or memory device. In a system that includesmultiple disks (e.g., multiple SSDs in a redundant RAID configuration)the memory controller may move the data to a different SSD. Once thedata is copied to the alternative location, the memory controllerrejuvenates the physical media of the cell group in question, at arejuvenation step 70. In alternative embodiments, the data stored in thecell group is not copied to another location. Instead, the memorycontroller refreshes the stored data in-place in order to prevent dataloss. An example method of this sort is described in FIG. 5 below.

Once the group of memory cells is rejuvenated, it remains in service. Inother words, memory controller 32 permits subsequent data storage inthis cell group. The method loops back to step 54 above, and the memorycontroller selects another cell group for evaluation.

In some embodiments, memory controller 32 rejuvenates a group of memorycells by inhibiting programming of the memory cells in the group for acertain time period. In Flash devices, refraining from programming thememory cells for a certain time period causes spontaneous de-trapping ofelectrons from the inter-cell oxide and/or bottom oxide, and thereforereduces the degradation of the physical memory media and improves thestorage quality of the memory cells. In some practical cases, refrainingfrom programming the memory cells for a period of between forty-eighthours and one week causes sufficient charge de-trapping, although anyother suitable time period can also be used.

In alternative embodiments, the memory controller may rejuvenate a groupof memory cells that was previously declared as faulty by re-attemptingto use (erase or program) the group. These attempts may be conductedperiodically, for example, or in response to events such as power-up.

Further alternatively, the memory controller may rejuvenate a group ofmemory cells by applying heating to the memory cells in the group.Heating the memory cells increases the likelihood (and rate) of chargede-trapping, and therefore considerably improves the storage quality ofthe memory cells' physical memory media. In some practical cases,heating the memory cells to a temperature on the order of 125° C. for aperiod of three hours causes sufficient charge de-trapping, although anyother suitable temperature and heating period can also be used. As canbe appreciated, heating the memory cells shortens the rejuvenationperiod considerably, in comparison with techniques that only inhibitprogramming of the memory cells. Typically, some of the physical mediadeterioration is irrecoverable, and therefore rejuvenation cannot beapplied infinitely. Nevertheless, the rate of degradation can be reducedconsiderably, thus providing a significant lifetime extension. Thememory system may comprise various means for applying heat to groups ofmemory cells.

FIG. 3 is a schematic, pictorial illustration of a memory system 80 thatemploys memory device heating, in accordance with an embodiment of thepresent invention. In system 80, memory controller 32 and multiplememory devices 28 are assembled on a printed circuit board 84. System 80comprises a heating plate 88, which is laid over some or all of memorydevices 28. The heating plate may comprise, for example, a thermocoupleplate or any other suitable type of heat source. Heating plate 88 iscontrolled by memory controller 32 via an interface 92.

In some embodiments, system 80 comprises a temperature sensor 96 that ismounted in the vicinity of memory devices 28. The temperature sensed bysensor 96 is read by the memory controller. In some embodiments, thememory controller can implement a closed loop that maintains the memorydevices at the appropriate temperature during the rejuvenation period.Alternatively, memory controller 32 may operate heating plate 88 in openloop. The memory controller may disconnect power supply from the memorydevices prior to heating. In some embodiments, once the rejuvenationcycle is completed, the memory controller tests the memory devices inorder to verify whether rejuvenation was successful.

FIG. 4 is a schematic, pictorial illustration of a memory system 100that employs memory device heating, in accordance with an alternativeembodiment of the present invention. Unlike system 80 of FIG. 3, inwhich heating is applied indiscriminately to multiple memory devices 28,in system 100 memory controller 32 applies heating selectively toindividual memory devices 28. System 100 comprises multiple localheating devices 108, each heating device attached to a respective memorydevice 28. Memory controller 32 can control each heating device 104individually using an interface 108, so as to apply heating selectivelyto only some of the memory devices.

Heating devices 104 may comprise, for example, thermocouple heaters,polyimide heaters, silicon heaters, mica heaters, or any other suitableheater type. Each heating device 104 typically applies heat on the orderof several watts locally to a given memory device 28. Devices 104 can beattached to memory devices 28 using any suitable means, such as bygluing. In some embodiments, the memory controller disconnects the powersupply from a given memory device before activating the correspondingheating device.

In alternative embodiments, memory controller 32 can apply heat to agiven group of memory cells by performing a stream of memory accessoperations (e.g., read or write operations) on the cell group inquestion or on a neighboring cell group. This technique is particularlysuitable for applying heat to specific word lines (rows of memory cellswithin a block) or to an entire memory block.

Further alternatively, the system can heat memory devices using theself-heating of system components. In an example embodiment, the memorydevices can be heated by preventing heat removal from the memorydevices. For example, airflow over the memory devices and/or heatdissipation from the memory devices can be blocked in order to increasethe device temperatures. As another example, the system can heat thememory devices by directing heat to these devices from other systemcomponents (e.g., from a Central Processing Unit—CPU). Furtheradditionally or alternatively, the memory system may use any othersuitable technique for applying heat to the memory cell groups in orderto rejuvenate the memory cell physical media.

In some embodiments, memory controller 32 rejuvenates a given group ofmemory cells by heating, but does not first copy the data from the cellgroup to another cell group that is not being heated. In this scenario,data errors may develop relatively quickly because of the hightemperature. For example, the rate at which the threshold voltages ofNAND cells drift increases with temperature. Therefore, heating thememory cells for a long time period may cause considerable thresholdvoltage drift and potential read errors. In order to prevent data loss,memory controller 32 refreshes the programming of the memory cells inthe group at a relatively high rate during the heating period. Thisrefreshing process is typically carried out by applying one or moreprogramming pulses to the memory cells, in order to compensate for thethreshold voltage drift. In the description that follows, the memorycontroller refreshes the programming of the memory cells in-place, i.e.,without copying the data to another location. In alternativeembodiments, however, the memory controller refreshes the programming ofthe memory cells by copying the data to another group of memory cells(which is being heated, as well).

FIG. 5 is a flow chart that schematically illustrates a method forrejuvenation and in-place data refreshing of analog memory cells, inaccordance with an embodiment of the present invention. The methodbegins with memory controller 32 assessing the health levels (storagequalities) of memory devices 28, at a health assessment step 120. Duringthis time, the memory controller may refresh the programming of thememory devices at a certain nominal rate, which is suitable for typicalthreshold voltage drifts that develop during normal operation.

At some stage, memory controller 32 identifies a faulty memory devicethat is to be rejuvenated, at a fault identification step 114. Thememory controller applies heat to the identified memory device, at aheating rejuvenation step 118. Any suitable heating scheme, such as theschemes of FIGS. 3 and 4 above, can be used for this purpose. During theheating period, the threshold voltages of the memory cells in the heatedmemory device drift at a higher rate than the normal rate, because ofthe high device temperature. Therefore, memory controller 32 refreshesthe programming of the memory cells in the heated memory device at arate that is higher than the nominal refresh rate used at step 110above. As noted above, refreshing may be performed in-place or bycopying the data to another location in the heated memory device.

When applying any of the rejuvenation techniques described herein, thememory controller may use any suitable management scheme and/or datastructures for tracking the health level of memory cell groups and/orfor selecting cell groups for rejuvenation. For example, the memorycontroller may hold a data structure that records the assessed healthlevel of each cells group (e.g., of each memory block, memory die,packaged memory device or multi-device memory assembly such as SSD). Inparticular, such a data structure may hold the status/condition,endurance and usage of each cell group.

In some embodiments, the health-related information may be soft ratherthan binary. For example, the memory controller may record whether aprogramming attempt failed completely, or whether the programmed levelsdeviated from the desired levels only slightly. As another example, whena group of memory cells failed to erase properly, the memory controllermay record the distance between the erased values of the memory cellsfrom the specified erase threshold.

The memory controller may initiate rejuvenation of a given cell groupwhen a given cell group is found to be faulty, or when the group'shealth level reaches a certain end-of-life condition. Selection of cellgroups for rejuvenation may be carried out, for example, by a backgroundtask whose priority is lower than that of memory access operations. Thebackground process can be repeated periodically, e.g., every few hoursor days, or in response to a certain condition or event such as a levelof memory access activity or power metric.

In some embodiments, when the memory controller intends to rejuvenate agiven cell group, it notifies higher levels (e.g., an operating systemor host) that this cell group is unavailable, and then proceeds withrejuvenation.

In some embodiments, when a certain cell group (e.g., die) is found tobe faulty, the memory controller moves the data stored in this group toalternative memory locations, and adds the faulty cell group to a listor pool of faulty groups. Copying can be performed, for example, bymapping the logical addresses of the data items stored in the faultygroup to new physical addresses. At a later time, e.g., using abackground task, the memory controller selects cell groups from thislist or pool and attempts to rejuvenate them. If rejuvenation of a givencell group is successful, the rejuvenated cell group is removed from thelist or pool of faulty groups. Higher layers may be notified that thecell group in question is operational.

Some of the disclosed techniques consume considerable amounts of power(e.g., heating) and/or computational and bandwidth resources (e.g.,refreshing). In these embodiments, rejuvenation is typically performedat relatively large intervals. For example, rejuvenation of faulty SSDs(or of individual memory devices in a SSD) can be performed duringscheduled system maintenance periods. In these embodiments, rejuvenationcan be performed using external heating means.

In some embodiments, memory controller 32 rejuvenates each memory device28 when the memory device reaches a certain level of use, e.g., acertain number of Programming and Erasure (P/E) cycles. Typically,however, the memory device should avoid rejuvenating a large number ofmemory devices at the same time. In an example embodiment, the memorycontroller rejuvenates different memory devices when they reachdifferent numbers of P/E cycles. For example, the memory device mayassign each memory device a number of P/E cycles in the range9000-11000, and rejuvenate each memory device when it reaches itsdesignated number of P/E cycles.

Rejuvenation Based on Prediction of Future Health Level of Memory CellGroups

In some embodiments, memory controller 32 attempts to rejuvenate a groupof memory cells whose current storage quality is still acceptable, butis predicted to deteriorate below the acceptable level shortly. Thus, insome embodiments the memory controller predicts the future health levelof a memory cell group based on characteristics that are currentlyobservable, and rejuvenates the memory cell group is the predictedstorage quality is too low.

FIG. 6 is a flow chart that schematically illustrates a method forrejuvenating analog memory cells, in accordance with an alternativeembodiment of the present invention. The description that follows refersto a memory block, although the method is equally applicable to variousother groups of memory cells.

The method begins with memory controller 32 observing characteristics ofa given memory block, at a current observation step 120. Based on theobserved characteristics, the memory controller predicts the futurehealth level of the block, at a prediction step 124. If the predictedhealth level is insufficient (e.g., lower than a predefined threshold),the memory controller rejuvenates the memory block, at a blockrejuvenation step 128. Any of the rejuvenation techniques describedherein can be used.

Memory controller 32 may use any suitable method for predicting thefuture health level of the memory block. In some embodiments, the memorycontroller calculates a metric denoted TM, which estimates an integralof the operating temperature of the block over time. The metric is thusgiven by TM=∫T(t)dt, wherein T(t) denotes the temperature of the memoryblock at time t. In a discrete-time implementation, the memorycontroller measures the temperature at t0-second intervals, and theintegral is estimated by TM=ΣP(t−t0)t0.

The memory controller may estimate T(t), for example, by directlymeasuring the temperature in the vicinity of the memory block. Thememory controller may use the readings of temperature sensor 96 of FIGS.3 and 4 for this purpose. Alternatively, the memory controller mayestimate T(t) indirectly by sensing the electrical current that isconsumed locally in the block, assuming that the device temperature isdependent on its power consumption. In some embodiments, the memorycontroller comprises a global timer for performing time measurements. Inthese embodiments, the memory controller can collect multipletime-temperature data points in order to compute the metric. Theintegration period is typically from the last programming operationuntil the present time, although other integration periods can also beused.

The memory controller then estimates the health level of the block basedon the above-described TM metric. For example, the metric value istypically indicative of the threshold voltage drift that is expected inthe block. If the memory controller measures a drift that isconsiderably different (typically faster) than the expected drift, theblock may be assumed to have poor health condition. Such cells may beregarded as candidates for rejuvenation.

The temperature integral metric TM described above provides an accuratemeasure of the threshold voltage shift in the memory cells, consideringthe actual temperature at which the memory cells operate. As such, thismetric can be used for various other purposes, which do not necessarilyinvolve rejuvenation of memory cells. For example, the memory controllermay estimate TM for a certain group of memory cells, and then adjust theread thresholds that are used for reading data from the memory cells inthe group based on the estimated TM value. The memory controller canthen read the threshold voltages of the memory cells in the group usingthe adjusted read thresholds, so as to reconstruct the data stored inthe cells. Since the optimal read threshold positions typically dependon the threshold voltage shift that develops in the memory cells, thistechnique enables accurate read threshold adaptation.

In some embodiments, the health level of a group of memory cells can beestimated based on statistical properties of the analog values (e.g.,threshold voltages) stored in the cells. The description that followsrefers to memory pages, although the disclosed techniques can be usedwith various other groups of memory cells. In these techniques,statistical properties of the cells' threshold voltages at a givenmeasurement time are used for estimating the number of read errors thiscell group will have at a certain future time. The future time can bedefined, for example, as the given measurement time, plus a certainnumber of P/E cycles, plus a certain retention period. Alternatively,any other suitable definition of the future time can also be used.

In an example embodiment, the following statistical properties areevaluated a-priori:

-   -   An empirical covariance between the current number of errors in        a page (i.e., at the measurement time) and the number of errors        in the page at the future time.    -   An empirical mean of the current number of errors in a page, at        the measurement time.    -   An empirical mean of the number of errors in a page at the        future time.    -   An empirical variance of the current number of errors in a page,        at the measurement time.

These properties can be evaluated, for example, by reading the cellthreshold voltages and counting errors over a large number of samplememory blocks. During operation, the memory controller can use theseproperties to estimate the number of errors a particular page isexpected to have at the future time. In an embodiment, the expectedfuture-time number of errors is estimated by:

${FutureTimeErrors} = {E{\left\{ {FutureTimeErrors} \right\}++}{\frac{{Cov}\left( {{FutureTimeErrors},{{Measurement}\mspace{14mu}{Time}\mspace{14mu}{Errors}}} \right)}{{Var}\left( {{Measurement}\mspace{14mu}{Time}\mspace{14mu}{Errors}} \right)} \cdot \left( {{{Measurement}\mspace{14mu}{Time}\mspace{14mu}{errors}} - {E\left\{ {{Measurement}\mspace{14mu}{Time}\mspace{14mu}{errors}} \right\}}} \right)}}$

Typically, the memory controller stores the relationship between thenumber of errors at the present measurement time and the predictednumber of errors at the future time, such as in one or more tables. thetables may hold relationships for several future time points. Using thisinformation, the memory controller may estimate the number of errorsthat a given page is expected to have at the future time point, anddecide whether or not to rejuvenate it based on the predicted number oferrors.

In an alternative embodiment, the number of future-time errors in a pageis estimated based on the width of the threshold voltage distribution ofthe cells associated with a given programming level. In this context,the width of a given programming level is defined as the size of thethreshold voltage interval, which includes a certain percentage (e.g.,99.7%) of the threshold voltages of the cells programmed to thatprogramming level.

In an example embodiment, a correlation coefficient between the width ofthe programming level and the number of errors the page will have at thefuture time point is estimated for each programming level in a page. Thecorrelation coefficient of the i^(th) programming level is given by:

$\rho_{i} = \frac{{Cov}\left( {{{width}\mspace{14mu}{of}\mspace{14mu}{level}\mspace{14mu} i},{FutureTimeErrors}} \right)}{\sqrt{{{Var}\left( {{width}\mspace{14mu}{of}\mspace{14mu}{level}\mspace{14mu} i} \right)} \cdot {{Var}({FutureTimeErrors})}}}$

Typically, the programming level having the largest correlationcoefficient is chosen, and the number of future-time errors is estimatedfrom the width of this programming level at measurement time. Theempirical variances and covariance in the above equation can beestimated, for example, by reading the cell threshold voltages andcounting errors over a large number of sample memory blocks. The memorycontroller typically stores the relationship between the presetdistribution width and the predicted number of errors at the future timepoint, e.g., in one or more tables. During operation, the memorycontroller uses these relationships to estimate the number of errors aparticular page is expected to have at the future time point. In anembodiment, the expected future-time number of errors is estimated by:

${FutureTimeErrors} = {{E\left\{ {FutureTimeErrors} \right\}} + {\frac{{Cov}\left( {{FutureTimeErrors},{{level}\mspace{14mu}{width}}} \right)}{{Var}\left( {{level}\mspace{14mu}{width}} \right)} \cdot \left( {{{level}\mspace{14mu}{width}} - {E\left\{ {{level}\mspace{14mu}{width}} \right\}}} \right)}}$

In many practical cases, the number of errors in a page has is affectedby both the shapes and the widths of the programming levels' thresholdvoltage distributions. In the early stages of the memory cells'operation, the number of errors is often more correlated with the shapesof the distributions than with their widths, because long distributiontails are often responsible for start-of-life errors. The widths of thedistributions at the future time point are typically correlated with thewidths of the respective distributions in earlier life stages. Thenumber of errors at the future time point is typically highly correlatedwith the distribution widths at end-of-life. Because of these typicalrelationships, it is often advantageous to estimate the number offuture-time errors in a given page based on (1) the widths of theprogramming level distributions in the page, and (2) the number oferrors in the page at measurement time.

Thus, in some embodiments, the memory controller estimates the number offuture-time errors in a given page (and therefore the health level ofthe page) by:

${FutureTimeErrors} = {{E\left\{ {FutureTimeErrors} \right\}} + {{Cxy} \cdot {Cyy}^{- 1} \cdot \left( {\underset{\_}{Y} - {E\left\{ \underset{\_}{Y} \right\}}} \right)}}$wherein $\underset{\_}{Y} = \begin{bmatrix}{{levels}\mspace{14mu}{width}\mspace{14mu}{at}\mspace{14mu}{Measurement}\mspace{14mu}{Time}} \\{{Measurement}\mspace{14mu}{Time}\mspace{14mu}{Errors}}\end{bmatrix}$ ${Cxy} = \begin{bmatrix}{{Cov}\left( {{{level}\mspace{14mu}{width}},{FutureTimeErrors}} \right)} \\{{Cov}\left( {{{Measurement}\mspace{14mu}{Time}\mspace{14mu}{Errors}},{FutureTimeErrors}} \right.}\end{bmatrix}$ ${Cyy} = \begin{bmatrix}{{Var}\left( {{level}\mspace{20mu}{width}} \right)} & \begin{matrix}{{Cov}\left( {{{level}\mspace{14mu}{width}},} \right.} \\\left. {{Measurement}\mspace{14mu}{Time}\mspace{14mu}{Errors}} \right)\end{matrix} \\\begin{matrix}{{Cov}\left( {{{level}\mspace{14mu}{width}},} \right.} \\\left. {{Measurement}\mspace{14mu}{Time}\mspace{14mu}{Errors}} \right)\end{matrix} & {{Var}\left( {{Measurement}\mspace{14mu}{Time}\mspace{14mu}{Errors}} \right)}\end{bmatrix}$

Constant Heating of Memory Devices

In some embodiments, system 20 can apply heat to one or more memorydevices, without explicitly identifying certain memory cell groups asrequiring rejuvenation. Constant heating of memory cells is advantageousin many practical scenarios, since it rejuvenates the physical media ofthe memory. In floating-Gate Flash devices, for example, constantheating increases the rate of charge de-trapping from tunnel oxide.

During operation of system 20, each memory device generates internalheat during its operation. The term “internal heat” refers to heat thatis generated by the memory device circuitry during operation, i.e., heatthat originates from the electrical power consumed by the memorycircuitry. In some embodiments, system 20 applies to one or more of thememory devices external heat, in addition to the internal heat generatedby the memory devices. The external heat increases the operatingtemperature of the memory devices, and causes accelerated chargede-trapping. As a result, the performance of the memory cells isimproved. The operating temperature is increased by the external heat,but typically remains within the operating temperature range specifiedfor the devices. For example, an increased temperature on the order of80-90° C., or any other suitable temperature, can be used.

System 20 may use any suitable mechanism for applying external heat tothe memory devices. For example, the system may use various types ofheaters coupled to the memory devices, direct heat from other systemcomponents, or use any of the heating schemes described in thisapplication. Note that a heater fitted inside the memory device packageis still considered a source of external heat, since its heat does notoriginate from the memory circuitry. Heating may be performed inopen-loop or in closed-loop. Heating may be performed continuously or atperiodic time intervals. In some embodiments, the system increases therate at which the data stored in the heated cells is refreshed, in orderto compensate for the accelerated threshold voltage drift caused by theheating (as described, for example, in FIG. 5 above).

Although the embodiments described herein mainly address rejuvenation ofnon-volatile solid-state memory cells, the methods and systems describedherein can also be used for memory cell rejuvenation in other memory andphysical media types, such as in Random Access Memory (RAM) and/or inmagnetic Hard Disk Drives (HDD).

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsub-combinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

The invention claimed is:
 1. A method for data storage, comprising: in amemory that includes multiple analog memory cells, storing data in agroup of the analog memory cells by writing respective analog storagevalues into the memory cells in the group; estimating an integral of atemperature of the memory cells in the group over time; setting at leastone read threshold for reading the memory cells in the groupresponsively to the estimated integral; and reading the analog storagevalues from the memory cells in the group using the at least one readthreshold, so as to reconstruct the stored data.
 2. The method accordingto claim 1, wherein estimating the integral comprises receiving readingsof the temperature from a temperature sensor, and computing the integralbased on the readings.
 3. The method according to claim 1, whereinestimating the integral comprises sensing a power consumption of thegroup of the memory cells, deducing the temperature from the sensedpower consumption, and computing the integral based on the deducedtemperature.
 4. The method according to claim 1, and comprisingassessing a health level of the memory cells in the group based on theintegral of the temperature.
 5. The method according to claim 1, whereinthe memory cells are fabricated using respective physical media, andcomprising identifying, based on the integral of the temperature, thatthe physical media of the group of the memory cells have deterioratedover time below a given storage quality level.
 6. The method accordingto claim 5, and comprising applying to the group a rejuvenation process,which causes the physical media of the memory cells in the group to meetthe given storage quality level.
 7. Apparatus for data storage,comprising: an interface for communicating with a memory that includesmultiple analog memory cells; and circuitry, which is configured tostore data in a group of the analog memory cells by writing respectiveanalog storage values into the memory cells in the group, to estimate anintegral of a temperature of the memory cells in the group over time, toset at least one read threshold for reading the memory cells in thegroup responsively to the estimated integral, and to read the analogstorage values from the memory cells in the group using the at least oneread threshold, so as to reconstruct the stored data.
 8. The apparatusaccording to claim 7, wherein the circuitry is configured to receivereadings of the temperature from a temperature sensor, and to computethe integral based on the readings.
 9. The apparatus according to claim7, wherein the circuitry is configured to sense a power consumption ofthe group of the memory cells, to deduce the temperature from the sensedpower consumption, and to compute the integral based on the deducedtemperature.
 10. The apparatus according to claim 7, wherein thecircuitry is configured to assess a health level of the memory cells inthe group based on the integral of the temperature.
 11. The apparatusaccording to claim 7, wherein the memory cells are fabricated usingrespective physical media, and wherein the circuitry is configured toidentify, based on the integral of the temperature, that the physicalmedia of the group of the memory cells have deteriorated over time belowa given storage quality level.
 12. The apparatus according to claim 11,wherein the circuitry is configured to apply to the group a rejuvenationprocess, which causes the physical media of the memory cells in thegroup to meet the given storage quality level.